1. Field of the Invention
The present invention relates to an impregnated cathode made of a porous element of a refractory metal and so on impregnated with an electron emitting substance (emitter) such as barium oxide (BaO) and a method of manufacturing the same, an electron gun and an electron tube.
2. Description of the Related Art
An impregnated cathode is used for an electron gun of a cathode-ray tube such as a picture tube and a display tube or an electron gun of an electron tube such as an image pickup tube and a high-frequency oscillator tube. Electrons (thermoelectrons) are emitted from the impregnated cathode.
The factors that determine the performance of such an impregnated cathode include a cathode cutoff voltage characteristic and a grid emission characteristic. It is important to reduce variations in the cathode cutoff voltage. The cathode cutoff voltage depends on the distance between the cathode and the first grid, the distance between the first and second grids, the thickness of the first and second grids, the aperture diameter of the first and second grids and so on. The grid emission is a symptom in which unintended emission of electrons occurs from excess barium and the like deposited on the grids (G1, G2 and so on). The grid emission is thus required to be reduced. In order to suppress unintended emission of electrons while maintaining the cathode cutoff voltage characteristic, it is required to increase the porosity in the electron emission region (working area) of the surface of the sintered porous element making up the impregnated cathode. At the same time, it is required to reduce the porosity rate or eliminate the pores in the other region so as to prevent the electron emitting substance for impregnation from being excessively vaporized through the region other than the electron emitting region.
Related-art impregnated cathodes are largely categorized into those of a single structure and those of a dual structure. The single-structure cathode only consists of a sintered porous element made of a refractory metal such as tungsten (W). The dual-structure cathode (such as the one disclosed in Japanese Patent Application Laid-open Sho 60-62034 [1985]) includes the electron emission region made of a porous sintered body and the region surrounding the electron emitting region made of a nonporous refractory metal. The two regions are fixed to each other through welding, for example.
However, such related-art impregnated cathodes have the following problems. It is difficult to make desired local variations of the porosity of the single-structure cathode made of a sintered porous element only. It is therefore extremely difficult to obtain the impregnated cathode whose impregnation amount of electron emitting substance is controlled as desired. If the cathode is impregnated with an ample amount of electron emitting substance so as to achieve a stable electron emission characteristic, barium (Ba) or barium oxide (BaO) as an electron emitting substance may evaporate and deposit onto the first or second grid during an operation of the cathode. As a result, the distance between the cathode and the first grid, and the distance between the first and second grids change and the cathode cutoff voltage drifts. Furthermore, it is impossible to reduce the grid emission.
On the other hand, in the dual-structure cathode made of a sintered porous element and a nonporous refractory metal, the refractory metal does not function as a storage of the electron emitting substance since the refractory metal is not capable of being impregnated with the electron emitting substance. Consequently, it is impossible to keep a sufficient amount of electron emission substance in the cathode for achieving a stable electron emission characteristic. The electron emission characteristic is thereby reduced and the life of the cathode is shortened. Another problem is that the manufacturing process is complicated and costs rise.
It is an object of the invention to provide an impregnated cathode and a method of manufacturing the same that suppress emission of unwanted electrons and particles generated from an excess electron emitting substance so as to achieve a steady electron emission characteristic and a long life of the cathode.
It is another object of the invention to provide an electron gun and an electron tube, each comprising such an impregnated cathode and having a steady characteristic.
An impregnated cathode of the invention is made of a conductive porous element having an electron emitting region and a peripheral region other than the electron emitting region and impregnated with an electron emitting substance in a surface thereof. The porous element has such a configuration that a porosity of part corresponding to the electron emitting region and a porosity of part corresponding to the peripheral region are different from each other. To be specific, the porosity of the part corresponding to the electron emitting region is greater than the porosity of the part corresponding to the peripheral region.
Another impregnated cathode of the invention has such a configuration that the porous element includes a nonporous surface in the peripheral region other than the electron emitting region.
Still another impregnated cathode of the invention has such a configuration that the porous element is made of a plurality of porous elements whose porosities are different from one another combined with one another, sintered and fixed to one another.
A method of manufacturing an impregnated cathode of the invention includes the steps of: separately fabricating a plurality of conductive porous elements whose porosities are different from one another; fixing the porous elements to one another and integrating the porous elements with one another; and having the porous elements each impregnated with an electron emitting substance.
Another method of manufacturing an impregnated cathode of the invention includes the steps of: separately fabricating a first conductive porous element and a second conductive porous element whose porosity is lower than that of the first porous element, the second porous element having a concave capable of accommodating the first porous element; having the concave of the second porous element filled with an electron emitting substance; and fixing the first porous element into the concave of the second porous element filled with the electron emitting substance and having the electron emitting substance diffused into the first and second porous elements.
Still another method of manufacturing an impregnated cathode of the invention includes the steps of: fabricating a conductive porous element including part corresponding to an electron emitting region and part corresponding to a peripheral region other than the electron emitting region in a surface of the porous element; grinding the part corresponding to the peripheral region of the porous element to form a nonporous surface; and having the porous element impregnated with an electron emitting substance.
Still another method of manufacturing an impregnated cathode of the invention includes the steps of: separately fabricating a first conductive porous element and a second conductive porous element whose porosity is lower than that of the first porous element, the second porous element having a concave capable of accommodating the first porous element; grinding a surface of the second porous element to form a nonporous surface; and fixing the first porous element into the concave of the second porous element and having the first and second porous elements each impregnated with an electron emitting substance.
Still another method of manufacturing an impregnated cathode includes the steps of: molding a plurality of conductive substances and fabricating a plurality of porous elements; provisionally sintering each of porous elements so that the shrinkage factors thereof are different from one another; sintering the porous elements combined with one another and fixing the porous elements to one another; and having the porous elements each impregnated with an electron emitting substance.
An electron gun of the invention includes a grid with an electron emission hole and an impregnated cathode made of a conductive porous element having an electron emitting region at least larger than the electron emission hole and a peripheral region other than the electron emitting region and impregnated with an electron emitting substance. The porous element of the impregnated cathode has such a configuration that a porosity of part corresponding to the electron emitting region and a porosity of part corresponding to the peripheral region are different from each other.
Another electron gun of the invention further includes a nonporous surface in the peripheral region in the surface of the porous element.
An electron tube of the invention comprises an electron gun including a grid with an electron emission hole and an impregnated cathode made of a conductive porous element having an electron emitting region at least larger than the electron emission hole and a peripheral region other than the electron emitting region and impregnated with an electron emitting substance. The porous element of the impregnated cathode has such a configuration that a porosity of part corresponding to the electron emitting region and a porosity of part corresponding to the peripheral region are different from each other.
Another electron tube of the invention further includes a nonporous surface in the peripheral region in the surface of the porous element.
In the impregnated cathode, the electron gun and the electron tube of the invention, the porosity of part corresponding to the electron emitting region of the porous element and the porosity of part corresponding to the peripheral region are different from each other. To be specific, the porosity of the part corresponding to the electron emitting region is greater than the porosity of the part corresponding to the peripheral region. As a result, the amount of the electron emitting substance contained in the part corresponding to the electron emitting region is different from the amount contained in the part corresponding to the peripheral region. Emission of electrons from the electron emitting region is therefore steadily performed. With the nonporous surface provided in the peripheral region other than the electron emitting region, evaporation of unwanted electron emitting substances and so on is suppressed and deposition thereof onto the grid is suppressed.
According to the other impregnated cathode, electron gun and electron tube of the invention, the nonporous surface provided in the peripheral region of the porous element suppresses emission of electron emitting substances from the peripheral region.
According to still the other impregnated cathode of the invention, the shrinkage factors of the plurality of porous elements are different from one another. As a result, no clearance is formed in the interface between the sintered porous elements during manufacturing. Emission of electrons from the electron emitting region is thereby steadily performed.
According to the method of manufacturing an impregnated cathode of the invention, the plurality of porous elements whose porosities are different from one another are fabricated in advance and the porous elements are then impregnated with the electron emitting substance. The impregnated cathode wherein the distribution of the electron emitting substance varies is thereby obtained.
According to the other method of manufacturing an impregnated cathode of the invention, the electron emitting substance fed into the concave of the second porous element is diffused into the first and second porous elements and the porous elements are impregnated with the electron emitting substance.
According to still the other method of manufacturing an impregnated cathode of the invention, the peripheral region of the porous element is ground to form the nonporous surface for suppressing emission of electron emitting substances and so on.
According to still the other method of manufacturing an impregnated cathode of the invention, the impregnated cathode is manufactured wherein the distribution of the electron emitting substance contained in the electron emitting region is different from that in the peripheral region and the nonporous surface is provided in the peripheral region.
According to still the other method of manufacturing an impregnated cathode of the invention, the plurality of porous elements whose shrinkage factors are different from one another are fabricated in advance and the porous elements are then fixed to one another by sintering.
Other and further objects, features and advantages of the invention will appear more fully from the following description.